1. Field of the Invention
The present invention relates to a novel thermotropic liquid crystal polyester, shaped articles comprising the same, and packaging materials and containers having high gas barrier properties.
2. Description of the Related Art
Polyesters, in particular polyethylene terephthalate (hereinafter sometimes referred to as "PET") are excellent in hygienic property, odor-keeping property, processability and like properties and are hence widely used as containers for seasonings such as soy sauce and sauce, soft drinks such as juice, cola and soda pop, draft beer, cosmetics, medicines and the like. It is expected that polyester bottles will be more widely used as replacement for glass bottles, since they are, in addition to the above features, lighter than glass and pressure-proof and of high gas barrier properties. For polyester containers, however, further improvement of gas barrier property is strongly desired since bottles for lager beer, wine and similar alcoholic drinks are stored for a long time and those for carbonated drinks are getting smaller in their size so that their surface areas per capacity are increasing, whereby there is more strictly required prevention of invasion of oxygen from outside or reduction in the amount of carbon dioxide gas dissipated outwardly. The gas barrier property of PET is very difficult to improve because it has already attained a considerably high level and that any improvement should not impair the processability into containers and the mechanical properties such as pressure-proofness. Various processes have been proposed to improve the gas barrier property of PET containers. For example a known process comprises coating polyvinylidene chloride or similar gas-barrier resins on the outer and inner surface of PET containers, and U.S. Pat. No. 4,980,211 discloses a multilayered structure of 2 to 5 layers utilizing a saponified ethylene-vinyl acetate copolymer. These processes however have disadvantages that additional equipment for coating or making multilayered container is required besides conventional molding and forming machines for polyesters and that the use of different polymers leads to readily delamination, with multilayered containers, or difficulty in the recovery or disposal by incineration of used containers. Japanese Patent Publication No. 33618/1978 and U.S. Pat. No. 4,398,642 propose a process which comprises producing containers from a composition obtained by previously blending polyester with nylon or like resins. Although the process enables containers to be produced with existing equipment, the obtained containers suffer a decrease in mechanical properties and have a disadvantage in recovery and re-use.
The use of what is known as thermotropic liquid crystal polymers, which are capable of forming optically anisotropic melt phase, has been proposed in recent years (See, for example, Japanese Patent Application Laid-opens Nos. 192762/1986, 119265/1987, 187033/1987, 45242/1989 and 288421/1 989). Also, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem. ), 30 (1), 3-4 (1989) reports that the thermotropic liquid crystal polymer obtained from 40 mol % polyethylene terephthalate and 60 mol % of 4-acetoxybenzoic acid gives a melt-extruded film having an oxygen permeability of 36 ml . 20 .mu.m/m.sup.2.day.atm. Further U.S. Pat. Nos. 3,778,410 and 3,804,805 disclose a process for producing a copolyester, which comprises reacting a polyester comprising repeating units represented by the formula --OC--R.sub.1 --CO--O--R.sub.2 --O-- wherein R.sub.1 represents a divalent alicyclic radical having 4 to 20 carbon atoms, a divalent aliphatic radical having 1 to 40 carbon atoms or a divalent aromatic radical having 6 to 16 carbon atoms with carbonyl linkages separated by at least 3 carbon atoms, and R.sub.2 represents a divalent aliphatic radical having 2 to 40 carbon atoms, a divalent alicyclic radical having 4 to 20 carbon atoms, a divalent aromatic radical having 6 to 20 carbon atoms or a divalent poly(alkylene oxide) radical having a molecular weight of 200 to 8,000; with an acyloxy aromatic carboxylic acid. These USP's mention substantially only acyloxy benzoic acids as examples of the acyloxy aromatic carboxylic acid.
However, the use of thermotropic liquid crystal polymers so far proposed for molding or forming shaped articles that act as gas-barrier materials has the following two principal problems.
Firstly, shaped articles obtained from thermotropic liquid crystal polymers so far proposed generally have high degree of crystallinity, large anisotropy in mechanical properties and low elongation, whereby their stretching is substantially impossible. It is very difficult to process these polymers into various gas-barrier shaped articles, such as film, sheet, bottles, cups, trays and bags.
To overcome the problem, Japanese Patent Application Laid-open No. 187033/1987 proposes a laminated and stretched shaped article comprising a layer of a thermotropic liquid crystal polyester and, on at least one surface thereof, a layer of polyester comprising polyethylene terephthalate component. The application discloses, with respect to the thickness ratio between the polyester (not exhibiting optical anisotropy) layer and the layer of the thermotropic liquid crystal polyester, that the thickness of the polyester layer is 50 to 98% of the total thickness of the laminated and oriented shaped article and that of the thermotropic liquid polyester layer is 2 to 50%, preferably 5 to 20% on the same basis, and describes that with a thickness of the thermotropic liquid crystal polyester exceeding 50% it is more difficult to stretch the laminated article than is the case with the usual polyester alone. Thermotropic liquid crystal polyesters having small anisotropy in mechanical properties have been also proposed. For example Japanese Patent Application Laid-open No. 28428/1985 discloses a thermotropic liquid crystal polyester comprising terephthaloyl units, 1,3-dioxyphenylene units and 2-substituted-1,4-dioxyphenylene units. Introduction of isoskeleton and substituents as in this proposal tends to improve processability of thermotropic liquid crystal polymer and to render it more ready, although not sufficient, to produce various shaped articles from the resulting polymers.
The second problem that arises when thermotropic liquid crystal polymers so far proposed are used for molding or forming gas barrier shaped articles is that these polymers are not always molded or formed into shaped articles having sufficient gas barrier properties. See, for example, the oxygen permeability of 36 ml . 20 .mu.m/m.sup.2.day.atm described in the afore-mentioned Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), 30 (1), 3-4 (1989), for a film of the thermotropic liquid crystal polymer obtained from 40 mol % of polyethylene terephthalate and 60 mol % of 4-acetoxybenzoic acid. This film cannot be said to be a high gas barrier material. Further a study made by the present inventors has revealed that films obtained from the thermotropic liquid crystal polyester described in Japanese Patent Application Laid-open No. 28428/1985 do not have a sufficiently high oxygen barrier property.
Japanese Patent Application Laid-open No. 68813/1987 discloses a copolyester obtained by reacting an acetoxy aromatic carboxylic acid mixture comprising p-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid with polyethylene terephthalate or polybutylene terephthalate, and describes that the product has higher flexural strength, flexural modulus and thermal deformation temperature than products obtained by reacting p-acetoxybenzoic acid alone as the acetoxy aromatic carboxylic acid to be used. This application however does not describe any packaging material or container comprising said copolyester, or teach that said copolyester has high gas barrier property, formability (stretchability), low temperature fluidity and similar excellent characteristics.